Turn your PMCA bluebird house into a House Sparrow nest-box trap. This insert trap thumb-screws into the inside of any wooden bluebird house that will accommodate its 3-3/8” x 6” dimensions, with the entrance holes aligned. Especially useful for people with bluebird trails or martin colony sites. This trap will catch a nest-building House Sparrow within minutes of installation in any sparrow-plagued bluebird house.